1. Field of the Invention
The present invention relates to a photo sensor in a photo diode on a semiconductor wafer and a method of forming the photo sensor.
2. Description of the Prior Art
The photo diode is a semiconductor device comprising a photo-conductivity cell and a junction diode, and is commonly used in photoelectric products, such as cameras and the photo sensors of scanners. The light-induced current of the photo diode represents a signal, whereas the current present in the absence of light represents noise. The photo diode processes signal data by using the magnitude of the signal-to-noise ratio. In the semiconductor industry, it is often desired to increase the light-induced current of the photo diode so as to increase the signal-to-noise ratio, and hence to enhance the contrast of the signal. The sensitivity of the photo diode would be enhanced and the quality of the photo diode would be improved.
Please refer to FIG. 1. FIG. 1 is a cross-sectional diagram of the structure of a prior art photo diode 10. A prior art photo sensor 20 in the photo diode 10 is positioned on a semiconductor wafer 11. The semiconductor wafer 11 comprises a silicon substrate 12 and a p-well 14 positioned on the silicon substrate 12. The photo diode 10 comprises an n-type metal oxide semiconductor (NMOS) transistor 16 positioned on the surface of the p-well 14, and a photo sensor 20 formed on the surface of the p-well 14 and electrically connected to the NMOS transistor. The semiconductor wafer 11 also comprises a field oxide layer 18 positioned on the surface of the p-well 14 that surrounds the photo sensor 20. The field oxide layer 18 acts as a dielectric insulating material to prevent short circuiting between the photo sensor and other units.
In the formation of the prior art photo sensor 20 of the photo diode 10, a high dosage of arsenic (As) atoms is used as the major dopant in an ion implantation process. This ion implantation process is performed to form an n-type doped region 22 on the surface of the p-well 14. A depletion region 24 for detecting the leakage current is formed along the PN junction between the doped region 22 and the adjacent p-type well 14. In FIG. 1, the area marked with slanting lines illustrates the depletion region 24.
The doped region 22 formed by the high dosage ion implantation process, and the p-well 14 that also has a high doping density, will both result in a narrower width of the depletion region 24, and will also decrease the real active region of the photo sensor 20. Therefore, the light-induced current sensed by the depletion region 24 is reduced. In addition, the interface of the doped region 22 and the p-well 14 under the field oxide layer 18 induces additional noise. As a result, the signal-to-noise ratio is lowered and the sensitivity of the photo sensor 20 is reduced.